The present invention relates generally to communications in a wireless local area network (WLAN) and, more particularly, to the dynamic adjustment of wireless communications protocol for meeting quality of service (QoS).
The Institute of Electronic and Electrical Engineers (IEEE) 802.11 standard for wireless LANs is becoming a popular mechanism for setting up networks in many industrial, office, home and medical environments. The WLAN is being preferred over the wired LANs due to ease of deployment and setting up of infrastructure as well as having mobility with network devices while having connectivity. The IEEE 802.11 protocol includes a media access control (MAC) layer for controlling wireless communication. The basic 802.11 MAC layer uses a Distributed Coordination Function (DCF) to share the wireless medium between multiple nodes in the WLAN. DCF relies on Carrier sense multiple access with collision avoidance (CSMA/CA) and optional 802.11 RTS/CTS (Request to Send/Clear to Send) to share the medium between nodes. The main limitation of the legacy 802.11 is that it cannot support Quality of Service differentiation among different types of traffic. That is, every type of traffic is treated with equal fairness in the network. This attribute is adequate for best effort traffic, but for delay and throughput sensitive traffic such as real time video, a prioritization framework is needed with high priority traffic getting a larger share of the shared wireless medium. (Note: Bandwidth is lowered primarily due to backoffs and not collisions. Collisions are prevented partially by CSMA and RTS/CTS).
To provide such QoS differentiation, the original 802.11 MAC defines another coordination function called the Point Coordination Function (PCF). When nodes are connected to the network through an Access Point (AP), APs send “beacon” frames at regular intervals (usually every 0.1 second). Between these beacon frames, PCF defines two periods: the Contention Free Period (CFP) and the Contention Period (CP). In CP, the DCF is simply used. In CFP, the AP sends Contention Free-Poll (CF-Poll) packets to each station, one at a time, to give them the right to send a packet. The AP is the coordinator. This allows for a better management of the QoS. However, as stated above, the PCF has limited support, does not define classes of traffic, and needs central coordination, which is not suitable in highly dynamic environments.
Since the legacy 802.11 does not have adequate support for QoS, a new standard called 802.11e has emerged which has prioritized traffic delivery for differentiating between traffic at different levels of criticality. The new standard achieves QoS by having separate MAC parameters for different service classes. That is, the 802.11e enhances the DCF and the PCF, through a new coordination function: the Hybrid Coordination Function (HCF). Within the HCF, there are two methods of channel access, similar to those defined in the legacy 802.11 MAC: HCF Controlled Channel Access (HCCA) and Enhanced Distributed Channel Access (EDCA). Both EDCA and HCCA define Traffic Classes (TC). For example, emails could be assigned to a low priority class, and Voice over Wireless LAN (VOWLAN) could be assigned to a high priority class.
With EDCA, 802.11e achieves Quality of Service differentiation by having different MAC parameters (TXOP, CW, AIFS, RL) for different traffic classes. A transmit opportunity (TXOP) is a bounded time interval during which a station can send as many frames as possible (as long as the duration of the transmissions does not extend beyond the maximum duration of the TXOP). If a frame is too large to be transmitted in a single TXOP, it is fragmented into smaller frames. Additionally, EDCA includes access categories and multiple independent back off entities for accessing each channel, including: a contention window size (CWmin[AC]) for each class, an Arbitration Interframe Space (AIFS[AC]), and a Frame Retransmission Limit (RL[AC]). Using different MAC parameters for low and high priority traffic class, the higher priority traffic is provided more opportunities to transmit than low priority traffic.
The HCCA (HCF (Hybrid Coordinator Function) Controlled Channel Access) is similar to PCF in legacy 802.11. However, in contrast to PCF (where the interval between two beacon frames is divided into two periods of CFP and CP), HCCA allows for CFPs being initiated at almost any time during a CP (called a Controlled Access Phase (CAP) in 802.11e), allowing more control for different traffic classes. A CAP is initiated by the Hybrid coordinator, (which is in fact the Access Point (AP)), whenever it wants to send a frame to a station, or receive a frame from a station, in a contention free manner. During a CAP, the Hybrid Coordinator (HC)—which is also the AP—controls the access to the medium. HCCA also defines new Traffic Streams (TS) in addition to Traffic Class (TC), which makes it possible to control QoS for traffic sessions at each node and affect individual packet queues. This means that the HC is not limited to per-station queuing and can provide a kind of per-session service. The HC can coordinate these streams or sessions in any fashion it chooses by giving information about the lengths of their queues for each Traffic Class (TC). The HC can use this info to give priority to one node over another, or better adjust its scheduling mechanism. Additionally, nodes are given a TXOP: they may send multiple packets in a row, for a given time period selected by the HC. With the HCCA, QoS-enabled nodes also have the ability to request specific transmission parameters (data rate, jitter, etc.), which allow applications like VoIP and video streaming to work more effectively on a Wi-Fi network.
Even though 802.11e can differentiate between the service classes, under standard operating conditions the 802.11e parameters are static in nature, meaning that it is not optimal under all network conditions. When network conditions in the WLAN change, the 802.11e parameters do not adapt to those changing conditions. Moreover, the default parameters for the different traffic classes are meant to specifically support Best Effort, Video and Voice traffic only. This makes the 802.11e default parameters unsuitable for some applications. For example, due to the static nature of its parameters, the 802.11e default parameters are unsuitable with respect to meeting the QoS requirements of medical devices used in patient monitoring in a hospital environment, where threshold levels of signal throughput and signal delay must be met.
Therefore, it would be desirable to design an apparatus and method that allows for the dynamic adjustment of MAC parameters for meeting a QoS requirement. It would also be desirable for the system and method to allow for automatically setting the MAC parameters in a distributed manner, using only local knowledge obtained at the individual node and for the MAC parameters to be based on an individual node's QoS requirements.